Absorption modulator switch utilizing gyromagnetic rotator split by resistive sheet



Sept. 10, 1963 F. REGGIAI ABSORPTION MODULATOR SWITCH UTILIZINGGYROMAGNETIC ROTATOR SPLIT BY RESISTIVE SHEET 3 Sheets-Sheet 1 Filed May12, 1961 INVENTOR Fran/r Reggia Sept. 10, 1963 F. REGGIA 3,103,639

ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BYRESISTIVE SHEET 5 Sheets-Sheet 2 Filed May 12, 1961 momkmmmo Q EE om n n8 S 8 ow 2 Omum v GE ISOLATION (db) WfVV Z an 6? Sept. 10, 1963 F.REGGIA 3,103,639

3 ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BYRESISTIVE SHEET 3 Sheets-Sheet 3 Filed May 12, 1961 O O PERM EABILITYINVENTOR Fran/r Reggia Ji l- TETMQ/ aai2,m 1+}? MAM,

United States Patent 0 3,103,639 ABSORPTION MODULATOR SWITCH UTILIZINGGYROMAGNETIC ROTATOR SPLIT BY RESIS- TIVE SHEET Frank Reggie, 3318 JonesBridge Road,

Chevy Chase 15, Md. Filed May 12, 1961, Ser. N 109,793 11 Claims. (Cl.ass-24.2 (Granted under Title 35, US. Code (1952), sec. 266) Thisapplication is a continuation-in-part of copending application SerialNo. 4,828, filed January 26, 1960, for Absorption Modulator Switch, nowabandoned.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment to me ofany royalty thereon.

This invention relates generally to a broadband absorptionmodulator-switch capable of providing very rapid switching of microwaveenergy.

Microwave devices used for switching or amplitude modulation ofmicrowave energy preferably should have a low insertion loss in thenormally On state and broad band operation. When the overall response ofthe device is independent of the frequency of the microwave energy whichis selectively switched, the operation may be termed broadband.

Other desirable characteristics include a matched input impedance forall values of applied magnetic field, high isolation substantiallyindependent of the magnitude of the applied magnetic field in the Offstate, reciprocal switching and modulating properties, low strengthmagnetic control fields, and small physical size.

An object of this invention is to provide an absorption modulator switchwhich has all of the above-mentioned characteristics.

' Another object of this invention is to provide a modulator switchhaving nearly constant electrical characteristics over a large X-bandfrequency range A further object is to furnish a reciprocal microwaveswitch which has a switching time of less than one microsecond.

Still another object of the present invention is to provide anabsorption modulator microwave switch in which the switching action isindependent both of the microwave input frequency and of the strength ofthe applied magnetic field.

An additional object is to provide a microwave switch having areciprocal response.

According to this invention, a magnetized ferrite rod which isincorporated in the waveguide section is split longitudinally and a thinresistive film is placed between the split sections of the ferrite. Whenthe ferrite is centrally located inside a standard rectangular waveguideexcited in its fundamental TE mode, with the resistive film in theferrite oriented perpendicular to the input electric field, theresistive film highly attenuates the orthogonal TE mode generated by thelongitudinally magnetized ferrite rod. The attenuation of thisorthogonal TE mode makes the occurrence of Faraday rotation impossiblesince the latter can exist only if the TE and the TE modes are presentsimultaneously. By the effective suppression of Faraday rotation in thepresent switch, nearly constant electrical characteristics over a verylarge X-band frequency range are achieved. The reciprocal switching ofmicrowaves is another result of suppressing Faraday rotation. It shouldbe understood that the term reciprocal is intended in its usual sense.It means that the electrical characteristics of the switch areindependent of the direction of propagation of microwave energy, as wellas the axial direction of the applied magnetic control field.

The specific nature of the invention, as well as other objects, uses andadvantages thereof, will clearly appear from-the following descriptionand from the accompanying drawing, in which:

FIG. 1 is a sectional side view of the modulator switch of thisinvention.

FIG. 2 is an end view of the device of FIG. 1.

FIG. 3a is a sectional side view of one embodiment of the resistive filmplaced between split sections of the ferrite rod.

FIG. 3b is a sectional side view of a preferred embodiment of theresistive film.

FIG. 4 is a plot of the electrical characteristics of the absorptionmodulator switch of this invention.

FIG. 5 illustrates the ferrite rod with the various electric andmagnetic fields shown therein.

FIG. 6 is .a plot of coupling coefficient K against the applied magneticfield H FIGS. 1 and 2 illustrate the microwave modulator switch 11 ofthis invention. 1 Therein, a standard rectangular waveguide section 12is excited in its fundamental TE mode. A microwave signal to be switchedis applied to one end of waveguide 12, exciting it in the aforesaidmode. A ferrite rod 13 is located on the central axis of waveguide 12.The rod 13 is split in half through itslongitudinal axis, forming thetwo sections 15 and 16 shown in FIG. 1. A resistive film 17, describedin detgfl subsequently, is located between ferrite sections 15 and.16,and is supported by the opposed planar surfaces of sections 15 and16. Asleeve 19 of polystyrene foam orother suitable plastic supports rod 13in a central position within waveguide 1'2 so that its longitudinal axisis concentric .with the longitudinal axis of the waveguide. Resistivefilm 17 is .thus placed along the waveguide axis in a planeperpendicular to the input R.-F. electric field. The sleeve .19 alsoserves to insulate the rod 13 from waveguide section 12. The ends offerrite rod 13 are tapered, respectively, as indicated at 24 and 25 inFIG. 1. Tapered ends 24 and 25 provide the impedance matching necessaryfor broadband operation.

-A solenoid coil 14 wrapped in convolutions around the outside ofwaveguide '12 selectively applies a D.-C. magnetic field-H shown in FIG.5, to \ferrite rod 13. Coil 14 is connected by a control switch 18 toany suitable source of electrical energy, such as battery 20 of FIG. 1.As set forth in more detail below, the state or condition of switch 11is controlled by controlling the application of current from source 20to coil 14. When current is applied to coil 14, longitudinallymagnetizing ferrite rod 13, the microwave switch 11 is in the Off state.When control switch 18 is open, as shown in FIG. 1, and no current ispresent in coil 14, microwave switch 1 1 is in its On or tnansmit-tingest-ate.

The operation of switch 11 may be explained in the following manner. Ingeneral, the internal R.-F. magnetic field h and the associated R.-F.flux density b induced by field h in a ferrite medium are related by atensor the subscripts referring to the three mutually perpendicular axesx, y and z. From the above expression, it has been shown that, in thecase of a magnetized but unsatu- See, for example, G. T. -Rlado,Electromagnetic Chameterization of Ferromagnetic Media, Transactions ofthe IRE '(PGAP), vol. AP-4, No. 3, pp. 512-525, July 1956. For thelatter case, a static D.-C. magnetic field H is applied along the z axisand n, K and ,u are the complex permeability components in the threemutually perpendicular directions. Expanding b: .7K 0 h:

y J' 0 i b, O 0 p, it, gives the following equations:

b h -jKh b =jKh h z=P-z x It can 'be seen from the first two equationsof (4) above that an applied R.-F. magnetic field h induces a componentb of the R.-F. flux density b which is proportional to ;1.. Due to theprecession of spinning electrons in the ferrite medium about the z axis,the magnetic field h also induces a component b in the y-directionproportional to K. Accordingly, ferrite rod 13 when magnetized Iby coil14 transfers portions of the applied microwave energy from the originalT E mode to a tightly coupled orthogonal TE mode. The latter modecorresponds to the flux density component b generated in the magnetizedtern'te 13.

The operation of absorption modulator switch 11 depends in part uponvariations of complex permeability component K with the static magneticfield H With a low-loss ferrite medium at microwave frequencies wherefield H is well below magnetic saturation, the real portion K' of thecomplex quantity K to a first approximation is where M, is themagnetization of the medium in the z direction, 7 is the :gyromagneticratio (for electrons equalling 2.8 megacycles per second per oersted,and w is the angular frequency of the incident microwaves in radians persecond. The variation of coupling coeflicient --K with the externalmagnetic field is plotted in FIG. 6, in accordance with experimentalresults. Only the real component is shown, the imaginary part beingsmall in comparison.

As seen from the curve in FIG. 6 and Equation 5 above, coeflicient -K iszero when no external magnetic field is applied by coil 14. Since,u'=,u.' =0.76 for this case, t and IL'Z being, respectively, the realportions of n and ferrite rod 13 exhibits a scalar permeability. Thisscalar permeability of rod 13 of less than unity at Xebfllld frequenciesand with no applied field, as opposed to the tensor permeability [,u] ofrod 13 when axially magnetized, is due to the gyromagnetic resonance ofthe election spins in the internal crystalline anisotropy field of theferrite. When it is desired to place switch 11 in its On state, controlswitch 18 is opened so that ferrite rod 13 is not magnetized. Theincident microwave energy propagated in the TE mode has its electricfield E directed perpendicular to the plane of the resistive 17, in themanner illustrated by FIG. 5. The input energy thus serves to apply anR.-F. magnetic field h of FIG. 5 to the ferrite rod :13. Since there isno field h in the y direction associated with the incident energy andsince com- 4 ponent K is negligible with the ferrite unmagnetized (itsmajor portion K'=0), Equations 4 reduce to:

x=I x b =0 z==/z z mode. While waves having an electric polarization ina plane parallel to resistive film 17 are dissipated thereby, the inputelectric field E is perpendicular to the plane of resistive film 17. Ittherefore does not induce currents in film 17 and suffers substantiallyno loss in propagating through waveguide section 12. The switch 11 iseffectively in its On state.

To turn the microwave switch 11 01f, control switch 18 is closed so thatcoil '14 produces the D.-C. magnetic field H within waveguide '12. H isapplied in the z direction, as seen in FIG. 5. Therefore, the ferrite 13exhibits the tensor permeability 1.] discussed supra. FIG. 6 shows thatfor any substantial applied field greater than 10 oersteds, the couplingcoetficient -K' has an appreciable magnitude of 0.5 or more.Accordingly, permeability component K is no longer negligible, as wasthe case with switch 18 open. Now Equations 4 take the form because theh component of the incident energy is zero, as before. FIG. 5illustrates the conditions present in ferrite rod 13. The incident R.-F.magnetic field h induces the flux density. b as before, and field hfurther induces a flux density b in the y direction within the ferritemedium 13 as shown by Equations 7. Flux density component b accounts forthe microwave energy converted by ferrite rod 13 to the tightly coupledorthogonal TE mode. This energy converted by ferrite 13 has an electricpolarization shown in FIG. 5 vector E 1 It is seen that electric vectorE perpendicular to b is parallel to the plane of the resistive film 17.The TE wave associated with b induces currents within the resistive film17 and is therefore immediately absorbed and dissipated within film 17.The end portion of ferrite rod '13 located at the input side ofwaveguide '12 receives the incident energy first and begins to generatethe orthogonal TE mode. That portion of the incident microwave energywhich the initially encountered section of fer-rite rod 13 converts tothe TE wave is substantially attenuated or damped as just described. Thebalance of the incident energy, which remains in the 'IE modecorresponding to the flux density component b of FIG. 5, is continuouslyand progressively attenuated as the signal propagates axially along rod13, even though its electric polarization is perpendicular to resistivefilm 17. This can be explained byconsidering the conditions at a pointfarther along rod 13 as shown in FIG. 5 by axes X and Y. While theoriginal R.-F. field of the incident energy was h at axes X and Y, it isreduced to a smaller strength h,, as illustrated by FIG. 5. Themicrowave R.-F. field is reduced from h to 11,, because the b componentof the original incident energy associated with I1 is dissipated in film17 as heretofore described. But the small fraction of the original inputsignal which reaches axes X and Y contains the R.-F. field h The latterfield again induces in the ferrite medium 13 two flux densities b and bthe enengy converted to the polarization of b being dissipated inresistive film 17. Therefore, by merely employing a ferrite rod 13 ofsufficient length, substantially all of the applied energy is attenuatedand switch I I is Off whenever a D.-C. field H is present.

i The microwave switch 11 of this invention has electrical switchingcharacteristics which are substantially independent of the microwavefrequency and of the strength of control field H Since this switch doesnot rely in its operation upon the Faraday rotational effect, itsinsertion loss and electrical isolation do not critically depend uponthe frequency of the incident energy or the control field magnitude, ashas been true of prior ferrite switching devices which make use of theFaraday rotation phenomenon. With the microwave switch 11 it is merelynecessary for the ferrite rod 13 to have sufiicient length andcross-sectional area to produce the desired degree of isolation at thelowest operating frequency expected. The cross-sectional area must belarge enough to concentrate the incident microwave energy in the ferritemedium at the lowest frequencies. As the input frequency increases,progressively shorter lengths of the ferrite rod are required for agiven degree of attenuation or damping. At higher frequencies, theferrite rod 13 is more effective in converting the applied energy to theorthogonal TE mode. Therefore, by making the ferrite rod-13 long enoughto isolate the lowest microwave frequency encountered, the rod has morethan enough length at higher frequencies. In this manner, independenceof the input frequency is obtained. i

The present switch is also reciprocal in nature and opends. The aboveswitch gives an isolation well 'over 60 film is properly adjusted,nearly zero phase shift occurs inch, while films 22 and 23 are 0.0004inch thick. Aquadag films 2 2 and '23 consist of a colloidal suspensionof graphite in water painted onto the opposite sides of dielectric film21. Film 21 may be any thin dielectric material such as Mylar.

A preferred type of resistive film '17 is shown in detail in FIG. 3b. Avery thin strip of high quality mica 26 serves as a base. Mica film 26has a thickness of 0.0005 inch and should be selected for uniformthickness. 'A very thin layer 27 of a pure metal such as chromium isdeposited by vaporization techniques upon film 26. The layer or film 27has a thickness of 0.0001 inch. Since the pure metal is in the form of avery thin film, it has a high resistance. The metal film 27 is coveredby a film 28 composed of a low loss dielectric serving as a protectivecoating to prevent oxidation of metal film 27. Film or layer 28 also hasa thickness of 0.0001 inch.

The curves of FIG. 4 illustrate the electrical characteristics ofparticular modulator switches constructed in accordance with thisinvention. The solid curves 31 and 32 illustrate, respectively, theinsertion loss in the Off state and the VSWR in the On state for amicrowave switch with a laminated resistive film 17 such as shown inFIG. 3a when operated at an input frequency of 9250 mc. The switch :11producing the response shown by curves 31 and 32 contains a splitmagnesium manganese ferrite rod 13 having a height of 0.280'inch, awidth of 0.300 inch, and a length of 4 inches including a one inch taperat each end for impedance matching (see FIGS. 1 and 2). This ferrite rodis supported by polystyrene foam insulation in a brass rectangularwaveguide section having an 0.40 inch by 0.90 inch internal crosssection. A switch so constructed provides nearly constant electricalcharacteristics over a frequency range from 7,500 megacycles to 10,500megacycles. The zero field insertion loss in the normally On state isaproximately 0.5 decibel and the isolation for the Off state exceeds 40decibels, as shown by curve 31. The input voltage standing wave ratio isless than 1.2 for all values of applied magnetic field, and theswitching time is less than one microsecond. A magnetic decibels at9,250 mc., as shown by curve 3 3 for control fields of30 'oersteds ormore. It has been found that this switch provides isolations greaterthan 60' decibels over a 2500 mc. bandwidth and as much as 55 dbisolation was obtained over a 3500 mc. bandwidth. 7

Finally, when the resistivity of the attenuating resistive at thedesired input frequency. Thus, amplitude modulation without any phasemodulation is possible with the absorption modulator switch of thisinvention.

It will be apparent that the embodiment shown is only exemplary and thatvarious modifications can be made in construction and arrangement withinthe scope of the invention as defined in the appended claims.-

I claim as my invention: I

1. An absorption modulator switch having broadband characteristics,comprising: a section of rectangular waveguide, a coil mounted aroundthe outside of said waveguide section for applying a longitudinalmagnetic field within said waveguide section, a ferrite rod insulat-ingly supported within said waveguide section so that thelongitudinal lELX S of said rod and saidwaveguide section are coaxial,said rod split longitudinally thereof forming two opposing sections, aresistive film lyingwholly in a plane which is oriented parallel to thelonger side walls of said waveguide section and located between said twoferrite sections, and a control circuit for selectively applying currentto said coil.

2. An absorption modulator switch as claimed in 1, wherein saidresistive filmcornprises a thin film of dielectric witlra coating of asuspension of minute graphite particles.

3. An absorption modulator switch as claimed in claim 1, wherein saidresistive film comprises a base layer of mica having a thin coating ofmetal vapor deposited thereon.

4. A reciprocal absorption modulator switch having broadbandcharacteristics, comprising: a rectangular waveguide excited in thefundamental TE mode, a coil around said waveguide for selectivelyapplying a longitudinal magnetic field to said waveguide, a ferrite rodinsulatingly supported within said waveguide for generating microwaveenergy in the TE mode, the longitudinal axis of said rod being coaxialwith the longitudinal axis of said waveguide, said rod beinglongitudinally splitso as to form two halves, and a resistive filmbetween said two halves for attenuating said TE mode energy saidresistive film lying wholly in a single plane which is oriented parallelto the longer side walls of said waveguide.

5. A reciprocal switch as claimed in claim 4, wherein said resistivefilm comprises athin film of dielectric with a coating of a suspension:of minute graphite particles.

6. A reciprocal switch as claimed in claim 4, wherein said resistivefilm comprises a base layer of mica having I a thin coating of metalvapor deposited thereon.

8. A reciprocal switch as claim in claim 7, wherein said resistive filmcomprises a thin film of dielectric with a coating of a suspension 0fminute graphite particles.

9. A reciprocal switch as claimed in claim 7, wherein said resistivefilm comprises a base layer of mica having a thin coating of metal vapordeposited thereon.

10. A reciprocal microwave switch comprising a rectangular waveguideexcited in the TE mode, coil means around said waveguide for selectivelyapplying a longitudinal magnetic field inside said waveguide, ferritemeans mounted along the longitudinal axis of said waveguide forselectively switching microwave energy inde pendently of the strength ofsaid magnetic field, said ferrite means comprising a ferrite rod splitin half and resistive attenuating means positioned perpendicular to theTE mode electric field and between the halves of said ferrite rod, andcontrol means for selectively applying current to said coil means.

11. A microwave switch comprising: a rectangular wave guide; means forselectively producing an electromagnetic field within said wave guide;and means tor reciprocally switching incident microwave energy withinsaid wave guide; said switching means comprising a ferrite 8 rod splitinto two sections and a resistive film positioned perpendicular to theshorter walls of said rectangular wave guide said resistive film beingpositioned between said two sections of said ferrite rod.

References Cited in the file of this patent UNITED STATES PATENTS2,802,183 Read Aug. 6, 1957 2,802,184 Fox Aug. 6, 1957 2,922,964 TurnerJan. 26, 1960 3,018,454 Sfernazza Jan. 23, 1962 3,022,475 Blasberg etlalFeb. 20, 1962 OTHER REFERENCES

1. AN ABSORPTION MODULATOR SWITCH HAVING BROADBAND CHARACTERISTICS,COMPRISING: A SECTION OF RECTANGULAR WAVEGUIDE, A COIL MOUNTED AROUNDTHE OUTSIDE OF SAID WAVEGUIDE SECTION FOR APPLYING A LONGITUDINALMAGNETIC FIELD WITHIN SAID WAVEGUIDE SECTION, A FERRITE ROD INSULATINGLYSUPPORTED WITHIN SAID WAVEGUIDE SECTION SO THAT THE LONGITUDINAL AXES OFSAID ROD AND SAID WAVEGUIDE SECTION ARE COAXIAL, SAID ROD SPLITLONGITUDINALLY THEREOF FORMING TWO OPPOSING SECTIONS, A RESISTIVE FILMLYING WHOLLY IN A PLANE WHICH IS ORIENTED PARALLEL TO THE LONGER SIDEWALLS OF SAID WAVEGUIDE SECTION AND LOCATED BETWEEN SAID TWO FERRITESECTIONS, AND A CONTROL CIRCUIT FOR SELECTIVELY APPLYING CURRENT TO SAIDCOIL.